Murine and human lymphocytes and plasma cells which synthesize immunoglobulins are being examined by immunologic, radio-labelling and cell culture techniques to determine the nature of the somatic genetic control of both immunoglobulin production and surface properties of these cells. The major techniques involve the adaptation of the cells to culture and the selection of clones which vary from the parent in the property under study. Using this approach we have obtained and are evaluating variants in both Ig synthesis and cell surface markers.